Java 8 stream.map()：线程“主”java.lang.IllegalStateException 中的异常：源已使用或关闭

Question

我有以下程序：

1. 以流的形式从文件中读取数据。
2. 在上述流上使用 stream.map() 创建一个新流并将其存储在 retVal 中。
3. 返回 retVal。
4. 使用 forEach 打印 Stream retval 的值，但我得到 Exception: IllegalStateException: source already used or closed
5. 对于函数workingPopulateFromFileAsStream，我已将其再次显式转换为流，然后它就可以正常工作了。

student.txt 的内容：

Name101,Last101
Name102,Last102
Name103,Last103
Name104,Last104
Name105,Last105
Name106,Last106
Name107,Last107
Name108,Last108
Name109,Last109
Name110,Last110

ClientMainApp.java：

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

class Student {
  String first_name;
  String last_name;

  public Student(String first_name, String last_name) {
    this.first_name = first_name;
    this.last_name = last_name;
  }

  public String toString() {
    return this.first_name + " " + this.last_name;
  }
}

public class ClientMainApp {
  public static void main(String... args) {
    String fileName = "student.txt";
    ClientMainApp obj = new ClientMainApp();

    //Working Code.
    Stream<Student> students_stream = obj.workingPopulateFromFileAsStream(fileName);
    System.out.println("working: Got Student Stream as " + students_stream);
    students_stream.forEach(System.out::println);
    System.out.println("==================================================");

    // Not Working Code.
    Stream<Student> students_stream2 = obj.populateFromFileAsStream2(fileName);
    System.out.println("notWorking: Got Student Stream2 as " + students_stream2);
    students_stream2.forEach(System.out::println);
  }

  private Stream<Student> workingPopulateFromFileAsStream(String fileName) {
    try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
      System.out.println("Working: Got File    Stream as " + stream);
      Stream<Student> retval =
          stream
              .map(
                  st -> {
                    ArrayList<String> record = new ArrayList<>(Arrays.asList(st.split(",")));
                    return new Student(record.get(0), record.get(1));
                  })
              .collect(Collectors.toList()).stream(); // This is the code making the difference.
      System.out.println("Working: Got Retval  Stream as " + retval);
      return retval;
    } catch (IOException e) {
      e.printStackTrace();
    }
    return null;
  }

  private Stream<Student> populateFromFileAsStream2(String fileName) {
    try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
      System.out.println("NotWorking: Got File    Stream  as " + stream);
      Stream<Student> retval =
          stream.map(
              st -> {
                ArrayList<String> record = new ArrayList<>(Arrays.asList(st.split(",")));
                return new Student(record.get(0), record.get(1));
              });
      System.out.println("NotWorking: got Retval  Stream  as " + retval);
      return retval;
    } catch (IOException e) {
      e.printStackTrace();
    }
    return null;
  }
}

我得到的结果是：

Working: Got File    Stream as java.util.stream.ReferencePipeline$Head@3e3abc88
Working: Got Retval  Stream as java.util.stream.ReferencePipeline$Head@214c265e
working: Got Student Stream as java.util.stream.ReferencePipeline$Head@214c265e
Name101 Last101
Name102 Last102
Name103 Last103
Name104 Last104
Name105 Last105
Name106 Last106
Name107 Last107
Name108 Last108
Name109 Last109
Name110 Last110
==================================================
NotWorking: Got File    Stream  as java.util.stream.ReferencePipeline$Head@7699a589
NotWorking: got Retval  Stream  as java.util.stream.ReferencePipeline$3@4dd8dc3
notWorking: Got Student Stream2 as java.util.stream.ReferencePipeline$3@4dd8dc3
Exception in thread "main" java.lang.IllegalStateException: source already consumed or closed
    at java.util.stream.AbstractPipeline.sourceSpliterator(AbstractPipeline.java:406)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
    at ClientMainApp.main(ClientMainApp.java:38)

在响应中，我们可以看到 retVal 的流对象是相同的，并且返回给调用函数，但我不知道为什么它说它已经被消耗了。

请注意，如果我在 populateFromFileAsStream2 内使用 forEach 循环，那么它可以正常工作。只有当函数返回值时，才会报错。

Answer 1

您的非工作版本基本上如下所示：

public Stream<String> foo(Path file) throws IOException {
  try (Stream<String> stream = Files.lines(file)) {
    return stream.map(...);
  }
}

您使用的是try-with-resources statement。这意味着一旦退出 try 块，Stream 就会关闭，这必须发生才能使方法返回。因此，您在从方法返回之前关闭流。也就是说，我认为你的错误根源在于：

1. 在上述流上使用 stream.map() 创建一个新流并将其存储在 retVal 中。

虽然map 确实会返回一个新的Stream 对象，但新对象仍然是同一概念流/管道的一部分。换句话说，关闭stream 引用与关闭retVal 引用相同。您的工作代码通过返回一个全新的管道来解决此问题，但代价是将整个文件缓冲到内存中。